The Prime Minister later apologised for the letter, which wrongly told people they had a month to leave the country.

But the citizens' groups claim the error has prompted serious concerns about the UK’s current proposal on “settled status".

"[The error] highlights very serious risks involved in the UK's proposal to replace existing rights of permanent residence with a new immigration status in UK law, even if some EU concepts are imported as part of their scheme," the letter said. "If serious errors like this can be made whilst the UK is still administering a system based on the EU's freedom of movement rights, what is likely to happen when it is running its own system, having "taken control again"?

"Additionally, we are concerned that the Home Office is planning to design a new system... before the negotiations on citizens rights have concluded."

The letter, signed by British in Europe chair Jane Golding and Costanza de Toma of the3million, added that the UK government had still not made it clear whether EU citizens would retain their automatic rights of appeal after Brexit.